The invention relates to a LC controllable oscillator (LCCO) comprising a voltage controlled oscillator (VCO), a first voltage controlled current source (VCCS) of a first type for supplying a current to the VCO, the VCO being realized with a first pair of VCCS of the first type coupled with a second pair of VCCS of a second type and a LC resonator adapted to be controlled for generating a periodical oscillation frequency which is controllable by a control signal (V), further comprising a first (SUP) conductor and a second (REF) conductor for connection to an external direct voltage source (VS).
The invention further relates to a module and an arrangement that uses the LCCO.
The following is defined in this description: if a VCCS of the first type is considered that sources it""s output current then a VCCS of the second type sinks it""s output current. Furthermore, if a VCCS of the first type is considered that sinks it""s output current then a VCCS of the second type sources it""s output current.
LC oscillators are well known circuits that are used in a large spectrum of applications for generating periodical signals. When they are used in high frequency applications as optical fiber networks, mobile telephony, transceivers and many others, they must provide, among other qualities, a good stability of the periodical signals versus temperature modification, they must be controllable over a wide frequency range of the periodical signals, and so on.
The LC oscillators are preferred in high frequency applications because of their frequency accuracy and reduced phase margin noise they exhibit. These LC oscillators have as their main components a pair of active devices, transistors for example and a LC tank circuit that determines their oscillation frequency.
Such an oscillator is disclosed in U.S. Pat. No. 5,959,504. It comprises a first pair of CMOS transistors with a tunable voltage applied to the back gate terminals for varying the parasitic capacitance of the transistor pair. The circuit further comprises a current generating means or a second transistor pair having a similar configuration but of opposite polarity to the first transistor pair that is connected to the first transistor pair and across an inductor. The frequency of the oscillation is determined by the product between the inductor inductance and the parasitic capacitance of the first pair of transistors. It should be pointed out here that the oscillation frequency is determined by technology dependent parameters as the CMOS parasitic capacitance and the frequency is controlled with a voltage applied at the back gate of the CMOS transistors, being dependent on a specific type of active device. Furthermore no measures are considered for thermally compensation of the oscillator.
It is therefore an object of the present invention to provide a LC oscillator with means to improve the temperature behavior of the frequency of the oscillation, the frequency of oscillation being determined independently of the technology.
In accordance with the invention this is achieved in a device as described in introductory paragraph characterized in that the LCCO further comprises a replica scaled bias module (RSBM) supplied from the external voltage source via the first conductor (SUP) and the second conductor (REF), the RSBM being conceived to generate a control signal (BIAS CONTROL) for controlling the supplied current delivered by the first VCCS to the VCO.
The LCCO according to the invention has the advantage of a frequency independent of technology and a better thermal stabilization.
In an embodiment of the invention the RSBM comprises a second, a third and a fourth VCCS, the second and the third VCCS being of the first type, the fourth VCCS being of the second type.
The VCCS used in the RSBM are replicas at a different scale of that used in the VCO, the currents circulating through them being proportional to each other. For example, if a current sourced or sinked by a VCCS is I0 then the replica scaled VCCS sources or sinks a current I0/m, m being the scale factor.
The BIAS CONTROL signal modifies in the same way the supply current of the VCO and it""s replica current in the RSBM module with the process and temperature variations. In this way the common mode voltage of the first pair of VCCS and of the second pair of VCCS is maintained constant. This improves the thermal stability and the phase noise margin of the LCCO.
In an embodiment the RSBM further comprises a current source of the first type coupled with a fifth VCCS of second type for providing a reference voltage to a first input of a differential voltage controlled voltage source (VCVS).
A second input of the VCVS is coupled with the fourth VCCS for supplying the signal (BIAS CONTROL) for controlling the supplied current in the VCO.
It should be pointed out here that the current source and the fifth VCCS may be regarded as a band gap reference voltage source that provides a better temperature behavior of the circuit. Furthermore, if the control signal V is generated using this band gap reference voltage, the oscillation frequency stability versus temperature is improved, too.
In a preferred embodiment of the invention, a LC tank circuit determines the frequency of oscillation that is controlled by an external control signal (V). The L and C components of the tank circuit have their inductance and capacitance much bigger than any other parasitic inductance and capacitance in the circuit and, as a matter of consequence, the oscillation frequency is determined independently of the technology.
It should be pointed out here that depending on the type the L and C components of the LC tank circuit it""s oscillation frequency can be controlled electrically, mechanically, thermally, optically.
Illustratively, all the previously described stages may be realized with transistors and LC tank resonators. In an embodiment all these transistors may be implemented in CMOS technology.
It is another object of the present invention to provide a module comprising the LCCO coupled with a phase shifter, controlled by the control signal (V), the phase shifter providing a first intermediate signal (S1) and a second intermediate signal (S2) to an adder (SUM), in which the intermediate signals S1 and S2 are added to each other for obtaining a signal (S) that is amplified by a first wide band amplifier (TIA) obtaining a first output signal (I), and to a subtraction circuit (DIF) where the intermediate signals S1 and S2 are subtracted from each other for obtaining a signal (D) that is amplified by a second wide band amplifier for obtaining a second output signal (Q).
The module is characterized in that the signals S1 and S2 are mutually phase shifted with 90 degrees.
The module is further characterized in that the output signals (I) and (Q) are periodical and mutually in quadrature.
Furthermore it is another object of the present invention to provide a communication arrangement for communicating via a bi-directional communication channel, characterized in that it comprises a receiver that comprises a data and clock recovery (DCR) circuit comprising a module as claimed in claim 6, the receiver being arranged for generating an output vector of signals by combining a received signal (IN) received from the channel with the periodical signals (I) and (Q), the arrangement further comprising an emission module for emitting an emission signal (OUT) to the channel, the emission module generating the emission signal by combining the periodical signals (I) and (Q) with an input signal vector (IN1) in a phase locked loop (PLL) circuit that contains the module.